## Parallel designs (total variance) [Power / Sample Size]

Dear Nav!

❝ I have data from a parallel pilot study and am using that to estimate the sample size for a pivotal study. The formula that I am using requires an estimate of the inter subject variability which I don't have. I have the total variability (sqrt(MSE)). Is there a way to estimate intersubject variability from a parallel design or would it be correct to use the total variability instead?

Hhm, yes you have the total variability (inter- + intra-subjects); the same will be true for your pivotal study. I don’t get your point of trying to estimate inter-subject variability from total. Which formula are you referring to?

❝ Would it be correct to estimate inter-subject var as sd/mean for test and reference separately and then take the average?

❝ And one last question, how correct is it to take 60% of inter subject (calculated as sd/mean) to be the intrasubject variability?

I don’t think so (both questions).

For an example please have a look at
Chow SC, Wang H. On Sample Size Calculation in Bioequivalence Trials. J Pharmacokin Pharmacodyn. 2001;28(2):155-69.
and Errata given at
J Pharmacokin Pharmacodyn. 2002;29(1):101-2.

You will also need two letters to the editor:
Hauschke D. A Note on Sample Size Calculation in Bioequivalence Trials. J Pharmacokin Pharmacodyn. 2002;29(1):89-94.
Blood P. Sample Size Calculation in Bioeqivalence Trials. J Pharmacokin Pharmacodyn. 2002;29(1):95-7.
H Wang H, SC Chow SC. Authors’ Response. J Pharmacokin Pharmacodyn. 2002;29(1):99.

Dif-tor heh smusma 🖖🏼 Довге життя Україна!
Helmut Schütz

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